clk: imx8mp: Remove non-existent IMX8MP_CLK_AUDIOMIX_PDM_ROOT

The TRM shows there is only one AUDIOMIX PDM Root Clock Select
register, and it's called IMX8MP_CLK_AUDIOMIX_PDM_SEL.  That
selector doesn't appear to have any more children and the
MICFIL driver can reference IMX8MP_CLK_AUDIOMIX_PDM_SEL
directly without the need for any other.  Remove this
errant clock, since it doesn't really exist.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230831044431.250338-3-aford173@gmail.com


Signed-off-by: Abel Vesa <abel.vesa@linaro.org>